During the 1990s, a new regulatory pattern in domestic environmental policy making emerged. This pattern is largely a result of policy diffusion. In the absence of formal obligations, regulatory instruments that have been communicated internationally and were already being practiced elsewhere were voluntarily emulated and adopted by policy makers. While the international promotion of regulatory instruments often facilitated their diffusion, the instruments’ characteristics determined the extent and speed by which regulatory instruments spread across countries. The voluntary adoption of regulatory instruments cannot be exclusively explained by the rational motivation of policy makers to improve effectiveness. In addition, they were motivated by concerns of legitimacy and perceived pressure to conform with international norms.